[DOC] JIT in CUDF

## Report needed documentation

**Report needed documentation**
CUDF has invested in JIT for expression evaluation, and it has proven to give significant speedups for complex expression evaluation whilst reducing global memory traffic.
The JIT development flow is presently complex and needs proper documentation of the entire process.
In addition, we would need a section explaining the tradeoffs and performance expectations for using JIT.

**Describe the documentation you'd like**
A documentation explaining JIT in the context of CUDF with the following sections:
- JIT developer flow: how to configure and add new JIT kernels
- Benchmarking with JIT (hot/cold cache behaviors)
- PCH setup and configuration for JIT kernels
- Configuration/Tuning options for JIT evaluation & cache management
- FAQ; Multi-container setup, Caching behaviour, How to Properly benchmark JIT, Slowdowns, etc.

**Steps taken to search for needed documentation**
List any steps you have taken: